1. Field of the Invention
The present invention relates to an optical recording medium and a process for preparing the same. More specifically, it relates to an optical recording medium comprising a substrate and an inorganic dielectric film thereon, and a process for preparing this kind of optical recording medium.
2. Related Background Art
In recent years, recording media which have a large recording capacity and which are capable of transferring data at a high speed have been demanded as memories for computers and memories for image information. In order to meet this demand, as the recording media, for example, magnetooptical recording media and phase change type recording media have been vigorously developed as optical recording media in which recording and reproduction are made by the use of laser beams.
That is, in the magnetooptical recording media, the recording and reproduction of information are made by the utilization of characteristics (a magnetic force effect) that when a deflected laser beam strikes the surface of a recording layer containing a magnetic material, the rotational direction of the deflected surface of the reflected laser beam depends upon the direction of the magnetization of the magnetic material.
On the other hand, in the phase change type recording medium, the recording and reproduction of information are made by using a reversible phase transition between an amorphous state and a crystalline state generated at the time of irradiating a recording layer containing, e.g., Te with a laser beam, and by using mutually different optical characteristics of the recording layer in these states.
In order for each of these optical recording medium to obtain a read signal having a larger C/N ratio, for example, an inorganic dielectric film can be interposed between a recording film and a substrate to amplify the read signal by the use of an optical interference effect (an enhancement effect). In addition, as shown in FIG. 4A, a recording film 41 can be sandwiched between inorganic dielectric films 42 and 43 to amplify the read beam and to protect the recording film 41.
Moreover, to further heighten the interference effect, as shown in FIG. 4B, using a reflective layer 44 mounted on the inorganic dielectric film 43 is also known.
As such an inorganic dielectric film, there has been used, for example, a thin film of Si.sub.3 N.sub.4, SiC, SiO, SiO.sub.2, amorphous Si (a-Si), AlN, Al.sub.2 O.sub.3, TiO.sub.2, Ta.sub.2 O.sub.5 or ZnS.
In this connection, performances of the magnetooptical recording medium or the phase change type optical recording medium using the inorganic dielectric film, for example, the C/N ratio of the read signal and the stability with time of the recording film depend largely upon not only the characteristics of the recording film but also those of the dielectric film, as described above. Therefore, in domestic and foreign research facilities, active research has now been carried out to develop dielectric materials having the large enhancement effect of the read signal and having excellent recording layer protection. Manufacturing techniques for these dielectric materials have also been studied. For example, as described in Japanese Patent Publication No. 2-15929, AlN or SiN is prepared in a nitrogen gas atmosphere by reactive sputtering with an Al target or an Si target, and in Japanese Patent Application Laid-open Nos. 3-66043, 3-69033 and 4-30343, it has been attempted to form the inorganic dielectric film for the optical recording medium by means of an ECR plasma CVD method.
In particular, the plasma CVD method improves film formation rate as compared with the sputtering process, and hence much attention has been paid to this plasma CVD method as a technique for obtaining high-quality optical recording media at a low cost.
However, if it is intended to achieve a high level of recording layer protection by using this plasma CVD film formation method, the inorganic dielectric film is inevitably stressed, and this tendency is more noticeable than when the film has been formed, heightening a plasma density to increase the film formation rate. In consequence, the inorganic dielectric film causes the optical recording medium to warp. Accordingly it is impossible to apply such an inorganic dielectric film to the optical recording medium. For this reason, when the inorganic dielectric film for the optical recording medium is formed by means of the plasma CVD method, the recording layer protection of the dielectric film has been heretofore decided in consideration of the internal stress of the inorganic dielectric film.
However, under circumstances where optical recording media having a higher quality is required, it is necessary that the recording layer protection is increased, while the increased internal stress in the inorganic dielectric film is inhibited as much as possible, even in the case where the inorganic dielectric film for the optical recording medium is formed by the plasma CVD method.